Liquid crystal displays (LCDs) are passive displays which depend upon external sources of light for illumination. They are manufactured as segmented displays or in one of two basic configurations. The substrate needs (other than being transparent and capable of withstanding the chemical conditions to which it is exposed during display processing) of the two matrix types vary. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second is extrinsic matrix or active matrix (AM) addressed, in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5-10 .mu.m.
Intrinsically addressed LCDs are fabricated using thin film deposition techniques at temperatures .ltoreq.350.degree. C., followed by photolithographic patterning. As a result, the substrate requirements therefor are often the same as those for segmented displays. Soda-lime-silica glass with a barrier layer has proven to be adequate for most needs. A high performance version of intrinsically addressed LCDs, the super twisted nematic (STN) type, has an added requirement of extremely precise flatness for the purpose of holding the gap dimensions uniform. Because of that requirement, soda-lime-silica glass used for those displays must be polished or, alternatively, a precision formed, barium aluminoborosilicate glass marketed by Corning Incorporated, Corning, N.Y., as Code 7059 may be used without polishing.
Extrinsically addressed LCDs can be further subdivided into two categories; viz., one based on MIM or amorphous silicon (a-Si) devices, and the other based on polycrystalline silicon (poly-Si) devices. The substrate requirements of the MIM or a-Si type are similar to the STN application. Corning Code 7059 sheet glass is the preferred substrate because of its very low sodium content, i.e., less than 0.1% Na.sub.2 O by weight, its dimensional precision, and its commercial availability. Devices formed from poly-Si, however, are processed at higher temperatures than those that are employed with a-Si TFTs. Substrates capable of use temperatures (taken to be 25.degree. C. below the strain point of the glass) of 600.degree.-800.degree. C. are demanded. The actual temperature required is mandated by the particular process utilized in fabricating the TFTs. Those TFTs with deposited gate dielectrics require 600.degree.-650.degree. C., while those with thermal oxides call for about 800.degree. C. Both a-Si and poly-Si processes demand precise alignment of successive photolithographic patterns, thereby necessitating that the thermal shrinkage of the substrate be kept low. Those temperatures have mandated the use of glasses exhibiting higher strain points than soda-lime-silica glass and Corning Code 7059 glass in order to avoid thermal deformation of the sheet during processing. As can be appreciated, the lower the strain point, the greater this dimensional change. Thus, there has been considerable research to develop glasses demonstrating high strain points so that thermal deformation is minimized during device processing at temperatures greater than 600.degree. C., and preferably, higher than 650.degree. C.
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four properties which have been deemed mandatory for a glass to exhibit in order to fully satisfy the needs of a substrate for LCDs:
First, the glass must be essentially free of intentionally added alkali metal oxide to avoid the possibility that alkali metal from the substrate can migrate into the transistor matrix;
Second, the glass substrate must be sufficiently chemically durable to withstand the reagents used in the TFT matrix deposition process;
Third, the expansion mismatch between the glass and the silicon present in the TFT array must be maintained at a relatively low level even as processing temperatures for the substrates increase; and
Fourth, the glass must be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.
That last requirement is most difficult to achieve inasmuch as it demands a sheet glass production process capable of producing essentially finished glass sheet, such as the overflow downdraw sheet manufacturing process described in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat. No. 3,682,609 (Dockerty). That process requires a glass exhibiting a very high viscosity at the liquidus temperature plus long term stability, e.g., periods of 30 days, against devitrification at melting and forming temperatures.
Corning Code 7059 glass, supra, is currently employed in the fabrication of LCDs. That glass, consisting essentially, in weight percent, of about 50% SiO.sub.2, 15% B.sub.2 O.sub.3, 10% Al.sub.2 O.sub.3, and 24% BaO, is nominally free of alkali metal oxides, and exhibits a linear coefficient of thermal expansion (25.degree.-300.degree. C.) of about 46.times.10.sup.-7 /.degree. C. and a viscosity at the liquidus temperature in excess of 600,000 poises (6.times.10.sup.-4 Pa.multidot.s). The high liquidus viscosity of the glass enables it to be drawn into sheet via the overflow downdraw sheet processing technique, but its relatively low strain point (.about.593.degree. C.) is adequate only for processing a-Si devices and not for poly-Si devices.
The glasses of U.S. Pat. No. 4,824,808, supra, were designed to meet the requirements for use in fabricating poly-Si devices, including the capability of being formed into sheet by the overflow downdraw sheet processing technique, and linear coefficients of thermal expansion as low as about 36.5.times.10.sup.-7 /.degree. C. (25.degree.-300.degree. C.), such as to closely match that of silicon, thereby enabling a silicon chip to De sealed directly thereon, but their strain points were less than 650.degree. C.
The glasses of U.S. Pat. No. 4,409,337 (Dumbaugh, Jr.) were also considered for LCD substrates, but their long term stability against devitrification was feared to be insufficient for their use in the overflow downdraw sheet processing technique.
The glasses of U.S. Pat. No. 5,116,787 (Dumbaugh, Jr.) are essentially free from alkali metal oxides and MgO and demonstrate strain points of 655.degree. C. and higher, with viscosities at the liquidus greater than 1.5.times.10.sup.5 poises (1.5.times.10.sup.4 Pa.multidot.s). Although designed for use in the overflow downdraw sheet processing technique, their long term stability against devitrification was found to be marginal when employed in the process, some crystallization being formed in the glass during manufacture.
U.S. Pat. No. 5,116,788 (Dumbaugh, Jr.) discloses other glasses exhibiting high strain points, i.e., greater than 675.degree. C., but having such relatively low viscosities at the liquidus temperature, viz., 20,000-200,000 poises (2,000-20,000 Pa.multidot.s), as to be subject to devitrification when formed utilizing the overflow downdraw sheet processing technique.